A slice-selective, highly under-sampled spiral or echo-planar imaging (EPI) k-space acquisition may be used for conventional two-dimensional magnetic resonance fingerprinting (MRF) acquisitions. Data may be acquired at 1000-2000 time points for a slice to be imaged. The data may be acquired during an acquisition time of approximately 10 milliseconds. Acquiring 1000-2000 data points using acquisition times of approximately 10 ms apiece results in an acquisition time of around 10-20 seconds per slice to be imaged using conventional MRF. Creating a high-resolution volumetric parameter map for slices that are 1 mm thick may involve imaging approximately 120 slices. This results in a total acquisition time of 20-40 minutes for conventional MRF. Faster imaging may be desired.
Conventional magnetic resonance imaging (MRI) may be constrained by the number of acquisition parameters that can be varied from signal acquisition period to signal acquisition period. Conventional multi-slice MRI may also be constrained by the number of acquisition parameters or acquisition conditions that can be varied from slice to slice during a single image acquisition. These constraints may lead to longer than desired acquisition times. Magnetic resonance fingerprinting (MRF) is not so constrained. MRF has more freedom based, at least in part, on how MRF produces nuclear magnetic resonance (NMR) that produces signal evolutions that include complex values with arbitrary phase relationships.
Characterizing resonant species using NMR can include identifying different properties of a resonant species (e.g., T1 spin-lattice relaxation, T2 spin-spin relaxation, proton density). Other properties like tissue types and super-position of attributes can also be identified using NMR signals. These properties and others may be identified simultaneously using MRF, which is described in Magnetic Resonance Fingerprinting, Ma D et al., Nature 2013:495(7440):187-192 and in U.S. Pat. No. 8,723,518, which is incorporated herein by reference.
Conventional magnetic resonance (MR) pulse sequences include similar repetitive preparation phases, waiting phases, and acquisition phases that serially produce signals from which images can be made. The preparation phase determines when a signal can be acquired and determines the properties of the acquired signal. For example, a first pulse sequence may produce a T1-weighted signal at a first echo time (TE) while a second pulse sequence may produce a T2-weighted signal at a second TE. These conventional pulse sequences typically provide qualitative results where data are acquired with various weightings or contrasts that highlight a particular parameter (e.g., T1 relaxation, T2 relaxation). These conventional pulse sequences tend to be similar to identical when applied to different slices of an object.
Unlike conventional MRI, MRF employs a series of varied sequence blocks that simultaneously produce different signal evolutions in different resonant species (e.g., tissues) to which the RF is applied. The term “resonant species”, as used herein, refers to an item (e.g., water, fat, tissue, material) that can be made to resonate using NMR. By way of illustration, when RF energy is applied to a volume that has bone and muscle tissue, then both the bone and muscle tissue will produce an NMR signal. However the “bone signal” and the “muscle signal” will be different and can be distinguished using MRF. The different signals can be collected over a period of time to identify a signal evolution for the volume. Resonant species in the volume can then be characterized by comparing the signal evolution to known evolutions. Characterizing the resonant species may include identifying a material or tissue type, or may include simultaneously identifying MR parameters associated with the resonant species. The “known” evolutions may be, for example, simulated evolutions or previously acquired evolutions. A large set of known evolutions may be stored in a dictionary.